Tanks for motor vehicles are generally provided with a system for controlling the emissions comprising an adsorption system made up of a “canister” or temporary accumulator for preventing fuel vapours from being dispersed into the atmosphere as they come out of the tank.
The canister generally comprises a housing connected to the tank and to the outside environment, and a main filter housed within the housing and traversed by the flow of fuel vapours emitted towards the outside environment.
The main filter is generally filled with granules of activated carbons which are capable of adsorbing hydrocarbons, which are the major constituents of fuel vapours at output from the tank.
Recently research has been directed towards systems for controlling evaporation that will enable better results to be obtained in terms of vapour adsorption and that will reduce further the diffusion of fuel vapours in the environment external to the vehicle.
Generally, within the canister vapour adsorption occurs on granules of activated carbons having a mean size of approximately 2 mm, which enable a high capacity for adsorption and at the same time ensure a low pressure drop, which is a measurement of the resistance that a material opposes to a given flow that traverses it. Basically, the pressure drop of activated carbons must be such that, during the phase of refuelling with the fuel gun inserted in the tank, the system of pressures will enable the fuel vapours to be drawn in instead of being emitted into the atmosphere, thanks to the system referred to as “on-board refuelling vapour recovery” (ORVR), and that consequently all the vapours generated by the handling of the petrol will have to pass within the evaporation-control system and hence through the activated carbons of the canister.
If the pressure drop is too high, the pressure of the system increases in a reverse direction, creating a resistance to further filling of the tank and hence generating frequent jerks or kick-back of the gun during supply with consequent pollution due to the emissions of fuel vapours.
The adsorption system must hence present a high capacity for adsorption and ensure a low pressure drop, but at the same time ensure also a high washing efficiency.
Recently, laws have been promulgated, which require that the admissable levels of emissions of unburnt hydrocarbons towards the outside environment will be reduced to values so low that it is necessary to act no longer only on the vapours coming from the tank that have not been adsorbed by the activated carbons of the canister, but also on the vapours of hydrocarbons released by the activated carbons as residue after the step of regeneration when the vehicle remains parked for a number of days. Said emissions are generally referred to also as “bleed emissions”.
Bleed emissions prevalently consist of short-chain hydrocarbon molecules, such as butane, which are entrapped within the granules of carbon of the adsorption system by weak Van der Waals forces, said molecules migrating slowly towards the area of the canister that is in contact with the air and then being dispersed into the surrounding environment.
The diffusion of said molecules depends upon Fick's law, and is a function of the diffusivity, of the type of molecule, of the area of diffusion, i.e., of the geometry of the canister, and upon the concentration of the molecules.
Carbons currently used in canisters are excellent in terms of adsorption capacity and enable good results in terms of pressure drop to be achieved but often do not enable low values of bleed emissions and at the same time a high washing efficiency.
In order to solve said problem, it has for example been proposed to fill the main filter with a first layer formed by first activated carbons with high adsorption power and a second layer of activated carbons with low adsorption power.
Alternatively, it has also been proposed to replace the second layer of activated carbons with an adsorption system made up of a supplementary filter that can be used in series with respect to the main one for abating further the content of harmful agents and in particular for reducing bleed emissions.
Said systems have not always proved very efficient and economically advantageous, and consequently there has been a continuous research directed towards better systems for controlling evaporation.